Can you paint parts of an image onto another image?
You need to use the texture painting tools:


Hit Tab to enter edit mode
Hit A to select everything and then UV unwrap you object Usmart UV project
Change your view to the UV image editor
Open your image that you wan't to paint on (You can create a new blank one as well)
Change your view back to 3D
Change into texture painting mode:
In the slots tab, set the painting mode to image, and select your image from the drop down.

Set up your brush
Configuration:


Set the Texture to you're image:

Set the brush mapping to tiled adjust the settings to make it look right.